LOSS OF PLASMID-MEDIATED OLIGOPEPTIDE TRANSPORT-SYSTEM IN LACTOCOCCI - ANOTHER REASON FOR SLOW MILK COAGULATION

Fast milk-coagulating (Fmc(+)) strains of lactococci are known to segr egate slow milk-coagulating (Fmc(-)) variants, which has been attribut ed to loss of proteinase (Prt) activity encoded by plasmid DNA. It was found that the Fmc(-) phenotype could also be due to loss of a plasmi d encoding an oligopeptide permease (Opp) system. In Lactococcus lacti s subsp. lactis (L. lactis) C2O, lactose metabolism (Lac) and Prt were linked to pJK550 and the Opp system to pJK430. In Lactococcus lactis subsp. cremoris SK11, known to possess Prt on a 78-kb plasmid, DNA seq uence analysis of a 7.4-kb region from the Lac plasmid, pSK11L, reveal ed that it possessed the Opp system. The Lac plasmid in L. lactis C2 e ncoded both the Prt and Opp systems. Fmc(-) derivatives of L. lactis C 2 were missing the prt genes and had Opp integrated into the chromosom e, possibly due to transposition events. Growth studies showed the Opp systems were functional and, in combination with Prt, produced the Fm c(+) phenotype. (C) 1996 Academic Press, Inc.